Loop heat pipe

ABSTRACT

A loop heat pipe includes an evaporator configured to vaporize a working fluid, a condenser configured to liquefy the working fluid, a liquid line connecting the evaporator and the condenser, and a vapor line connecting the evaporator and the condenser. The evaporator, the condenser, the liquid line, and the vapor line are formed by stacking a lowermost metallic layer, an uppermost metallic layer, and an intermediate layer set formed of a plurality of metallic layers, which is provided between the uppermost metallic layer and the lowermost metallic layer. The evaporator, the liquid line, the condenser, and the vapor line form a loop-shaped flow path through which the working fluid flows, and a portion of the flow path is formed in the intermediate layer set.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of patent application Ser. No.15/156,869 filed on May 17, 2016, which is a continuation application ofInternational Application PCT/JP2013/083504 filed on Dec. 13, 2013 anddesignated the U.S., the entire contents of which are incorporatedherein by reference.

FIELD

The embodiments discussed herein are related to a loop heat pipe.

BACKGROUND

With the advent of an advanced information society, mobile electronicdevices such as smartphones and tablet computers are widely used. Sincethe mobile electronic devices become thinner to be easily carried, it isdifficult to provide a fan and a blower to cool heat generatingcomponents such as a CPU (Central Processing Unit).

As an example of a method of cooling the heat generating components,there is a method of transferring heat of the heat generating componentsto the outside using a metallic plate or a heat spreading sheet withhigh thermal conductivity. In this method, however, heat transfer islimited by the thermal conductivity of the metallic plate and the heatspreading sheet. For example, graphite sheets used as the heat spreadingsheets have a thermal conductivity of about 500 W/mK to 1500 W/mK. Withthe thermal conductivity on that order, it is difficult to cool the heatgenerating components when the amount of heat of the heat generatingcomponents becomes large.

To deal with this problem, heat pipes are under consideration in orderto cool actively the heat generating components.

The heat pipe is a device which transfers heat using a phase change of aworking fluid, and has a thermal conductivity higher than that of theheat spreading sheet described above. For example, a heat pipe with adiameter of 3 mm exhibits a high thermal conductivity of about 1500 W/mKto 2500 W/mK.

Several types of heat pipes are available. A loop heat pipe includes anevaporator configured to vaporize the working fluid with use of the heatfrom the heat generating components and a condenser configured to coolthe thus-vaporized working fluid into a liquid. The evaporator and thecondenser are connected to each other via a liquid line and a vapor linewhich form a loop-shaped flow path, and the working fluid flows throughthe flow path in a one-way direction.

Since the working fluid of the loop heat pipe flows in the one-waydirection as described above, the loop heat pipe gives lower resistanceto the working fluid than the conventional heat pipe in which theworking fluid in a liquid phase and the vapor thereof flow back andforth through the line, and thus achieves efficient heat transfer.

Note that technologies related to the present application are disclosedin Japanese National Publication of International Patent Application No.2011-530059, Japanese Laid-open Patent Publication No. 2004-3816, andJapanese Laid-open Utility Model Publication No. 05-25164.

SUMMARY

According to one aspect discussed herein, there is provided a loop heatpipe including: an evaporator configured to vaporize a working fluid; acondenser configured to liquefy the working fluid; a liquid lineconnecting the evaporator and the condenser; and a vapor line connectingthe evaporator and the condenser. The evaporator, the condenser, theliquid line, and the vapor line are formed by stacking a lowermostmetallic layer, an uppermost metallic layer, and an intermediate layerset formed of a plurality of metallic layers, which is provided betweenthe uppermost metallic layer and the lowermost metallic layer. Theevaporator, the liquid line, the condenser, and the vapor line form aloop-shaped flow path through which the working fluid flows, and aportion of the flow path is formed in the intermediate layer set.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a loop heat pipe used in a study;

FIG. 2 is an enlarged plan view of an evaporator and its vicinity of theloop heat pipe used in the study;

FIG. 3 is a schematic plan view of an electronic device according to thepresent embodiment;

FIG. 4 is a cross-sectional view of an evaporator and its vicinity of aloop heat pipe according to the present embodiment;

FIG. 5 is a cross-sectional view taken along line I-I in FIG. 3;

FIG. 6 is a schematic plan view of the loop heat pipe in which anuppermost metallic layer is omitted in the present embodiment;

FIG. 7 is a cross-sectional view of a liquid line of the loop heat pipeaccording to the present embodiment;

FIG. 8 is a plan view illustrating pores in the second to fifth layersof the metallic layer in the loop heat pipe according to the presentembodiment;

FIG. 9 is a plan view schematically lustrating a position of each of thepores in the stacked metallic layers in the present embodiment;

FIG. 10 is a plan view illustrating another example of a size of thepore in the present embodiment;

FIG. 11A is a schematic plan view illustrating an example of the size ofthe pore in the evaporator according to the present embodiment;

FIG. 11B is a schematic plan view illustrating an example of the size ofthe pore in the liquid line according to the present embodiment;

FIG. 12A is a schematic plan view of a loop heat pipe according to acomparative example;

FIG. 12B is a schematic plan view of the loop heat pipe according to thepresent embodiment;

FIG. 13 is a graph obtained by measuring a temperature of vaporimmediately after emission from the evaporator in each of thecomparative example and the present embodiment;

FIG. 14 is a (first) plan view of the metallic layer used in themanufacture of the loop heat pipe according to the present embodiment;

FIG. 15 is a (second) plan view of the metallic layer used in themanufacture of the loop heat pipe according to the present embodiment;

FIG. 16 is a cross-sectional view of the loop heat pipe in the course ofmanufacturing according to the present embodiment; and

FIG. 17 is a cross-sectional view drawn based on an SEM (ScanningElectron Microscope) image of a portion that corresponds to the porousbody of the loop heat pipe according to the present embodiment.

DESCRIPTION OF EMBODIMENTS

Prior to a description of the present embodiment, studies conducted bythe inventor of the present application will be described.

FIG. 1 is a schematic diagram of a loop heat pipe used in the study.

The loop heat pipe 1 is placed in a mobile electronic device 2 such as asmartphone, and includes an evaporator 3 and a condenser 4.

A vapor line 5 and a liquid line 6 are connected to the evaporator 3 andthe condenser 4. The vapor line 5 and the liquid line 6 form aloop-shaped flow path through which working fluid C flows. In addition,a heat generating component 7 such as a CPU is fixed on the evaporator3, and the heat of the heat generating component 7 generates vapor Cv ofthe working fluid C. The vapor Cv flows through the vapor line 5 to thecondenser 4, and then is liquefied in the condenser 4. Thus, the heatgenerated from the heat generating component 7 is transferred to thecondenser 4.

FIG. 2 is an enlarged plan view of the evaporator 3 and its vicinity.

As illustrated in FIG. 2, a wick 10 is placed in the evaporator 3.Ideally, a portion of the wick 10 near the liquid line 6 is impregnatedwith the working fluid in a liquid phase.

When such a state is maintained, capillary force from the wick 10 actson the working fluid C in the liquid phase, and the capillary forceresists against the vapor Cv of the working fluid C. Therefore, theworking fluid C in the liquid phase can be expected to function as acheck valve which prevents the vapor Cv from flowing back from the vaporline 5 to the liquid line 6.

However, according to the studies conducted by the inventor of thepresent application, it was found that the vapor Cv flows back in theevaporator 3 when the loop heat pipe 1 is made thinner.

One conceivable reason for this is that the thinning increases apressure loss in the vapor line 5 so much that the flow of the vapor Cvstagnates in the vapor line 5, and the vapor Cv is no longer capable ofpushing out the working fluid C in the liquid phase in the condenser 4(see FIG. 1) to the liquid line 6.

Another conceivable reason is that the heat generating component 7 heatsthe liquid line 6 and thereby vaporizes part of the working fluid C inthe liquid line 6, which in turn causes a backward flow of the vapor Cvdescribed above. Note that a phenomenon in which the liquid line 6 isheated by the heat generating component 7 in this manner is alsoreferred to as heat leak.

When the vapor Cv flows back as described above, heat transferperformance of the loop heat pipe significantly decreases, and itbecomes difficult to cool the heat generating component 7.

The present embodiment, which is capable of preventing a backward flowof the working fluid even when thinned, will be described.

Present Embodiment

FIG. 3 is a schematic plan view of an electronic device according to thepresent embodiment.

The electronic device 20 is a mobile electronic device such as asmartphone and a tablet computer, and includes a housing 21 and a loopheat pipe 22 housed in the housing 21.

The loop heat pipe 22 includes an evaporator 23 configured to generatethe vapor Cv of the working fluid C, and a condenser 24 configured toliquefy the working fluid C. In addition, a vapor line 25 and a liquidline 26 are connected to the evaporator 23 and the condenser 24, and thelines 25 and 26 form a loop-shaped flow path through which the workingfluid C flows.

FIG. 4 is a cross-sectional view of the evaporator 23 and its vicinity.

As illustrated in FIG. 4, the evaporator 23 is fixed on a circuit board31 with screws 33. A heat generating component 32 such as a CPU ismounted on the circuit board 31, and a surface of the heat generatingcomponent 32 is in contact with the evaporator 23. This makes itpossible for the heat generating component 32 to vaporize the workingfluid C in the evaporator 23.

The kind of the working fluid C is not particularly limited. However, inorder to cool the heat generating component 32 efficiently with latentheat of vaporization, it is preferable to use such a fluid as theworking fluid C that has high vapor pressure and high latent heat ofvaporization. Such a fluid includes ammonium, water, fluorocarbon,alcohol, and acetone.

FIG. 5 is a cross-sectional view taken along line I-I in FIG. 3, andcorresponds to a cross-sectional view of the vapor line 25.

As illustrated in FIG. 5, the vapor line 25 is formed by stacking sixmetallic layers 34, for example. Each metallic layer 34 is made of, forexample, copper with excellent thermal conductivity and is bonded to oneanother by diffusion bonding. In addition, a thickness of each of themetallic layers 34 is about 0.1 mm to 0.3 mm.

Note that stainless layers or magnesium alloy layers may be employed asthe metallic layers 34 in place of the copper layers. It should be notedthat materials of all of the metallic layers 34 are preferably the samein order to excellently bond the metallic layers 34 by the diffusionbonding.

Moreover, the number of the metallic layers 34 to be stacked is notlimited to the above. The number of the metallic layers 34 to be stackedmay be equal to or less than five, or equal to or greater than seven.

Then, the metallic layers 34 define a base surface 34 w, a ceilingsurface 34 v, and walls 34 x of the vapor line 25.

In addition, a pillar 35 is provided in the vicinity of the center ofthe vapor line 25. The pillar 35 supports the ceiling surface 34 v ofthe vapor line 25 from below, and prevents collapse of the vapor line 25due to pressing force exerted in the process of stacking the metalliclayers 34. This ensures the provision of a flow path 34 y which allowsthe vapor Cv to flow in the vapor line 25 even when the loop heat pipe22 is thinned. As a consequence, the vapor Cv can flow smoothly in theloop heat pipe 22.

Note that the evaporator 23, the condenser 24, and the vapor line 25 arealso formed by stacking the metallic layers as described above.

FIG. 6 is a schematic plan view of the loop heat pipe 22 from which theuppermost metallic layer 34 is omitted.

Dimensions of the loop heat pipe 22 are not particularly limited. Inthis example, a width W1 of the vapor line 25 is about 8 mm, and a widthW2 of the liquid line 26 is about 6 mm.

In addition, a planar shape of the pillar 35 is a linear shape extendingalong the vapor line 25. This allows the vapor Cv to flow smoothly inthe vapor line 25 along the pillar 35. Here, a width W3 of the pillar 35is about 1 mm.

A flow path 24 x for the working fluid C is provided in the condenser24. Each ends of the flow path 24 x are connected to the vapor line 25and the liquid line 26 respectively. The pillar 35 is also provided inthe flow path 24 x and can prevent collapse of the flow path 24.

In addition, a porous body 36 is placed in the liquid line 26. Theporous body 36 extends along the liquid line 26 toward the vicinity ofthe evaporator 23. The working fluid C in the liquid phase in the liquidline 26 is guided to the evaporator 23 with the assistance of capillaryforce generated in the porous body 36.

Consequently, even when the vapor Cv attempts to flow back in the liquidline 26 due to heat leak from the evaporator 23 and the like, it ispossible to push back the vapor Cv with the assistance of theaforementioned capillary force exerted by the porous body 36 on theworking fluid C in liquid phase. Thus, the backward flow of the vapor Cvcan be prevented.

The porous body 36 is placed in the evaporator 23.

In the evaporator 23, a portion of the porous body 36 near the liquidline 26 is impregnated with the working fluid C in the liquid phase.Here, the capillary force exerted by the porous body 36 on the workingfluid C serves as pumping force which circulates the working fluid Cthrough the loop heat pipe 22.

Moreover, this capillary force resists against the vapor Cv in theevaporator 23. Thus, it is possible to suppress a backward flow of thevapor Cv toward the liquid line 26.

Note that in the liquid line 26, an inlet 34 c is formed through whichthe working fluid C is poured. The inlet 34 c is sealed with anunillustrated seal member, and the inside of the loop heat pipe 22 isthus kept sealed.

FIG. 7 is a cross-sectional view of the liquid line 26, and correspondsto a cross-sectional view taken along line II-II in FIG. 6.

As illustrated in FIG. 7, the entire porous body 36 is provided in acolumnar shape in the cross-sectional view. Thus, the porous body 36 canprevent collapse of the liquid line 26 due to pressing force exertedwhen the metallic layers 34 are stacked.

In addition, multiple pores 34 a are provided in a portion of themetallic layers 34 that corresponds to the porous body 36. Adjacentpores 34 a communicate with each other, and fine channels are thusdefined by the pores 34 a. The channels extend three-dimensionally inthe porous body 36, and the working fluid C permeates the channelsthree-dimensionally with the assistance of the capillary force.

Here, the position of the porous body 36 in the liquid line 26 is notparticularly limited. However, it is preferable to provide the porousbody 36 away from the wall 34 x of the liquid line 26, as illustrated inFIG. 7. Thereby, a fine channel 34 y through which the working fluid Cflows is formed between the porous body 36 and the wall 34 x, whichmakes it easier for the working fluid C to flow in the liquid line 26.

FIG. 8 is a plan view illustrating the pores 34 a of the second to thefifth layers of the metallic layers 34.

In the example of FIG. 8, a shape of each of the pores 34 a is circular.Then, these pores 34 a are provided at intersections of imaginary linesL that are perpendicular to one another.

Here, a diameter R of the pore 34 a and a distance D between adjacentpores 34 a may be optimized by taking into consideration an amount ofthe transferred heat and the heat transfer distance required for theloop heat pipe 22, a height of the each of the vapor line 25 and theliquid line 26, and the like.

Moreover, the shape of the pore 34 a is not limited to a circle. Thepore 34 a may be formed in any shape such as an ellipse and a polygon.

In addition, the positions of the pores 34 a are different among thesecond layer to the fifth layer of the metallic layers 34.

FIG. 9 is a plan view schematically illustrating the positions of thepores 34 a when the metallic layers 34 are stacked.

Since the positions of the pores 34 a are different among the metalliclayers 34 as described above, the pores 34 a overlap one another in aplan view, as illustrated in FIG. 9.

It is not particularly limited how the pores 34 a overlap each other. Inthis example, in the two metallic layers 34 vertically adjacent to eachother, at least a part of the pore 34 a in one metallic layers 34overlaps the pore 34 a in the other metallic layer 34. This allows theworking fluid C to flow three-dimensionally through the pores 34 a inthe vertically adjacent metallic layers 34.

Although FIG. 9 illustrates the case where the size of all the pores 34a in all of the metallic layers 34 is the same, the size of the pores 34a is not limited to this.

FIG. 10 is a plan view illustrating another example of the size of thepore 34 a.

In the example of FIG. 10, in the two metallic layers 34 verticallyadjacent to each other, a diameter R1 of the pore 34 a in one metalliclayers 34 is different from a diameter R2 of the pore 34 a in the othermetallic layer 34.

By making the size of the pores 34 a different between the verticallyadjacent metallic layers 34 in this manner, the capillary force exertedby the porous body 36 on the working fluid C can be adjusted

Meanwhile, as described above, the porous body 36 is also provided inthe evaporator 23. The size of the pores 34 a in the evaporator 23 maybe different from that in the liquid line 26 as described below.

FIG. 11A is a schematic plan view illustrating an example of the size ofthe pore 34 a in the evaporator 23, and FIG. 11B is a schematic planview illustrating an example of the size of the pore 34 a in the liquidline 26.

In the example of FIGS. 11A and 11B, a diameter R3 of the pore 34 a inthe evaporator 23 is made smaller than a diameter R4 of the pore 34 a inthe liquid line 26.

In this case, the working fluid C can flow smoothly through the largerpores 34 a in the liquid line 26, and can be transferred rapidly to theevaporator 23. In the evaporator 23, on the other hand, the workingfluid C in the liquid phase serves as check valves with the assistanceof the capillary force exerted by the smaller pores 34 a. Hence, it ispossible to effectively suppress a backward flow of the vapor Cv asdescribed above.

Next, a study conducted by the inventor of the present application willbe described.

FIGS. 12A and 12B are schematic plan views of loop heat pipes used inthis study.

FIG. 12A is a schematic plan view of a loop heat pipe according to acomparative example. In this comparative example, the porous body 36 isnot provided in the liquid line 26, and the pillar 35 also is notprovided in the vapor line 25.

On the other hand, FIG. 12B is a schematic plan view of the loop heatpipe according to the present embodiment. In the present embodiment, asdescribed above, the porous body 36 is provided in the liquid line 26,and the pillar 35 is provided in the vapor line 25.

In this study, a measurement point P for measuring a temperature wasprovided in each of the loop heat pipes of the comparative example andthe present embodiment. Then, a temperature of the vapor Cv immediatelyafter emission from the evaporator 23 was measured at each measurementpoint P.

Measurement results are given in FIG. 13. A horizontal axis of FIG. 13represents the elapsed time from the time point when an unillustratedheat source started to heat the evaporator 23. Meanwhile, a verticalaxis of FIG. 13 represents a temperature of the measurement point P. Inaddition, water was used as the working fluid C in both the comparativeexample and the present embodiment.

In the comparative example, as illustrated in FIG. 13, the temperaturesuccessfully increases during the time period of the elapsed time of 0seconds to 600 seconds, but decreases sharply when the elapsed timeexceeds 800 seconds. The reason for this is considered as follows. Thatis, during the time period of 0 seconds to 600 seconds, the vapor Cv ofthe working fluid C generated in the evaporator 23 passed themeasurement point P and thus the temperature rose. In contrast, when thetime exceeds 800 seconds, the vapor Cv flowed back through the vaporline 25, and the generating point P is no longer heated by the vapor Cv.

On the other hand, the present embodiment did not exhibit a decrease intemperature as observed in the comparative example. Hence it followsthat the vapor Cv did not flow back through the vapor line 25, and themeasurement point P was heated by the vapor Cv at all times.

From these results, it was confirmed that it is possible to suppress thebackward flow of the vapor Cv by providing the porous body 36 in theliquid line 26 as in the present embodiment.

Next, a method of manufacturing the loop heat pipe 22 according to thepresent embodiment will be described.

FIG. 14 and FIG. 15 are plan views of the metallic layer 34 used formanufacturing the loop heat pipe 22.

FIG. 14 is a plan view of the metallic layer 34 used for the uppermostlayer and the lowermost layer of the loop heat pipe 22. FIG. 15 is aplan view of the metallic layer 34 provided between the uppermost layerand the lowermost layer.

The metallic layers 34 illustrated in FIG. 14 and FIG. 15 can befabricated, for example, by wet-etching a copper layer of thickness ofabout 0.1 mm and patterning the copper layer into a predetermined shape.

In addition, in this wet etching, openings 34 z are formed in themetallic layers 34 as illustrated in FIG. 15. The openings 34 z haveshapes that corresponds to the evaporator 23, the condenser 24, thevapor line 25, and the liquid line 26 of the loop heat pipe 22.

Further, a part 36 a of the porous body 36 is provided in a portion ofthe metallic layers 34 that corresponds to the liquid line 26. Then, themultiple pores 34 a are formed by the aforementioned wet etching in thepart 36 a and in the evaporator 23.

On the other hand, a part 35 a of the pillar 35 is provided in a portionof the metallic layers 34 that corresponds to the vapor line 25.

Here, the parts 35 a and 36 a are connected to the metallic layers 34 bybridges 34 y, and are thus prevented from detaching from the metalliclayer 34. In order to prevent the bridges 34 y from closing the vaporline 25 and the liquid line 26, it is preferable that the positions ofthe bridges 34 y be made different among the metallic layers 34.

In addition, in the portion of metallic layer 34 that corresponds to theliquid line 26, an inlet 34 c is provided for pouring the working fluidC.

Then, as illustrated in FIG. 16, the multiple metallic layers 34described above are stacked. FIG. 16 is a cross-sectional view of theportion that corresponds to the liquid line 26 after the stacking.

In this stacking, the metallic layers 34 illustrated in FIG. 14 aredisposed as the uppermost layer and the lowermost layer, and themultiple metallic layers 34 illustrated in FIG. 15 are arranged betweenthe uppermost layer and the lowermost layer.

Subsequently, the metallic layers 34 are pressed against each otherwhile heating the metallic layers 34 at around 900° C. Thereby, themetallic layers 34 are bonded together by diffusion bonding. At thistime, since the porous body 36 functions as a pillar as described above,it is possible to prevent collapse of the liquid line 26 due to thepressing.

Thereafter, the liquid line 26 is evacuated through the inlet 34 c (seeFIG. 15) with an unillustrated vacuum pump. Thereafter, water as theworking fluid C is poured into the liquid line 26 through the inlet 34c, and then the inlet 34 c is sealed.

Thus, the fabrication of the loop heat pipe 22 according to the presentembodiment is completed.

FIG. 17 is a cross-sectional view drawn based on an SEM (ScanningElectron Microscope) image of the portion corresponding to the porousbody 36 in the loop heat pipe 22.

As illustrated in FIG. 17, the metallic layers 34 are integrated withone another as a result of the diffusion bonding, and the interfaces ofthe metallic layers 34 are disappeared.

According to the present embodiment described above, by providing theporous body 36 of a columnar shape in the liquid line 26, the workingfluid C in the liquid line 26 is guided to the evaporator 23 by thecapillary force generated in the porous body 36. Thus, it is possible tosuppress the backward flow of the working fluid C from the evaporator 23to the liquid line 26.

the porous body 36 provided in the columnar shape can prevent the liquidline 26 from collapsing.

Moreover, the multiple metallic layers 34 are stacked to manufacture theloop heat pipe 22. Therefore, it is possible to thin the loop heat pipe22 to such a degree that the loop heat pipe 22 can be housed in asmartphone, a tablet computer, and the like.

All examples and conditional language recited herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A loop heat pipe comprising: an evaporatorconfigured to vaporize a working fluid; a condenser configured toliquefy the working fluid; a liquid line connecting the evaporator andthe condenser; and a vapor line connecting the evaporator and thecondenser; wherein the evaporator, the condenser, the liquid line, andthe vapor line form a loop-shaped flow path through which the workingfluid flows; the evaporator, the condenser, the liquid line, and thevapor line are formed by stacking a lowermost metallic layer, anuppermost metallic layer, and an intermediate layer set provided betweenthe uppermost metallic layer and the lowermost metallic layer, theintermediate layer set being formed of a plurality of metallic layersincluding a first metallic layer and a second metallic layer provided onthe first metallic layer; the first metallic layer includes a pair offirst walls forming walls of the evaporator, the condenser, the liquidline, and the vapor line, a first opening penetrating the first metalliclayer, and a second opening penetrating the first metallic layer; thesecond metallic layer includes a pair of second walls forming walls ofthe evaporator, the condenser, the liquid line, and the vapor line, athird opening penetrating the second metallic layer, and a fourthopening penetrating the second metallic layer; the first opening and thethird opening are disposed to coincide with each other in a plan view;the second opening and the fourth opening are disposed to partiallyoverlap each other in a plan view, such that the second opening is incommunication with the fourth opening; the first opening and the thirdopening form a first portion of the flow path; the second opening andthe fourth opening form a second portion of the flow path; and thecommunication between the second opening and the fourth opening isnarrower than the communication between the first opening and the thirdopening.
 2. The loop heat pipe according to claim 1, wherein the secondopening and the fourth opening are provided in the liquid line.
 3. Theloop heat pipe according to claim 1, wherein a size of the secondopening differs from a size of the fourth opening.
 4. The loop heat pipeaccording to claim 2, wherein the first metallic layer further includesa fifth opening penetrating the first metallic layer; the secondmetallic layer further includes a sixth opening penetrating the secondmetallic layer; the fifth opening and the sixth opening are provided inthe evaporator; and the fifth opening and the sixth opening are smallerthan the second opening or the fourth opening.
 5. The loop heat pipeaccording to claim 1, wherein the pair of first walls includes a firstside wall, and a second side wall that is positioned opposite the firstside wall; the pair of second walls includes a third side wall, and afourth side wall that is positioned opposite the third side wall; thefirst metallic layer further includes a seventh opening penetrating thefirst metallic layer; the second metallic layer further includes aneighth opening penetrating the second metallic layer; the seventhopening and the eighth opening are disposed to coincide with each otherin a plan view; the first opening and the third opening are disposedadjacent to the first side wall and the third side wall, respectively;and the seventh opening and the eighth opening are disposed adjacent tothe second side wall and the fourth side wall, respectively.
 6. The loopheat pipe according to claim 5, wherein, in the vapor line, a pillarthat supports the uppermost metallic layer is formed by a portion of thefirst metallic layer disposed between the first opening and the seventhopening, and a portion of the second metallic layer disposed between thesecond opening and the eighth opening.
 7. A loop heat pipe comprising:an evaporator configured to vaporize a working fluid; a condenserconfigured to liquefy the working fluid; a liquid line connecting theevaporator and the condenser; and a vapor line connecting the evaporatorand the condenser; wherein the evaporator, the condenser, the liquidline, and the vapor line form a loop-shaped flow path through which theworking fluid flows; the vapor line is formed by stacking a lowermostmetallic layer, an uppermost metallic layer, and an intermediate layerset provided between the uppermost metallic layer and the lowermostmetallic layer, the intermediate layer set being formed of a pluralityof metallic layers including a first metallic layer and a secondmetallic layer provided on the first metallic layer; the first metalliclayer includes a pair of first walls forming walls of the vapor line, afirst opening penetrating the first metallic layer, and a second openingpenetrating the first metallic layer; the second metallic layer includesa pair of second walls forming walls of the vapor line, a third openingpenetrating the second metallic layer, and a fourth opening penetratingthe second metallic layer; the first opening and the third opening aredisposed to coincide with each other in a plan view; and the firstopening, the second opening, the third opening, and the fourth openingform a portion of the loop-shaped flow path connecting the evaporatorwith the condenser.
 8. The loop heat pipe according to claim 7, whereinthe pair of first walls includes a first side wall, and a second sidewall that is positioned opposite the first side wall; the pair of secondwalls includes a third side wall, and a fourth side wall that ispositioned opposite the third side wall; the first opening and the thirdopening are disposed adjacent to the first side wall and the third sidewall, respectively; and the second opening and the fourth opening aredisposed adjacent to the second side wall and the fourth side wall,respectively.
 9. The loop heat pipe according to claim 8, wherein, inthe vapor line, a pillar that supports the uppermost metallic layer isformed by a portion of the first metallic layer disposed between thefirst opening and the second opening, and a portion of the secondmetallic layer disposed between the third opening and the fourthopening.
 10. A loop heat pipe comprising: an evaporator configured tovaporize a working fluid; a condenser configured to liquefy the workingfluid; a liquid line connecting the evaporator and the condenser; and avapor line connecting the evaporator and the condenser; wherein theevaporator, the condenser, the liquid line, and the vapor line form aloop-shaped flow path through which the working fluid flows; theevaporator is formed by stacking a lowermost metallic layer, anuppermost metallic layer, and an intermediate layer set provided betweenthe uppermost metallic layer and the lowermost metallic layer, theintermediate layer set being formed of a plurality of metallic layersincluding a first metallic layer and a second metallic layer provided onthe first metallic layer; the first metallic layer includes a pair offirst walls forming walls of the evaporator, the condenser, the liquidline, and the vapor line, a first opening penetrating the first metalliclayer, and a second opening penetrating the first metallic layer; thesecond metallic layer includes a pair of second walls forming walls ofthe evaporator, the condenser, the liquid line, and the vapor line, athird opening penetrating the second metallic layer, and fourth openingpenetrating the second metallic layer; the first opening and the thirdopening are disposed to coincide with each other in a plan view; thesecond opening and the fourth opening are disposed to partially overlapeach other in a plan view, such that the second opening is incommunication with the fourth opening; the first opening and the thirdopening form a first portion of the flow path; the second opening andthe fourth opening form a second portion of the flow path; and thecommunication between the second opening and the fourth opening isnarrower than the communication between the first opening and the thirdopening.
 11. The loop heat pipe according to claim 10, wherein a size ofthe second opening differs from a size of the fourth opening.
 12. Theloop heat pipe according to claim 10, wherein the liquid line is alsoformed by stacking the lowermost metallic layer, the uppermost metalliclayer, and the intermediate layer set provided between the uppermostmetallic layer and the lowermost metallic layer, the intermediate layerincluding the first metallic layer and the second metallic layerprovided on the first metallic layer; the first metallic layer furtherincludes a fifth opening penetrating the first metallic layer; thesecond metallic layer further includes a sixth opening penetrating thesecond metallic layer; the fifth opening and the sixth opening areprovided in the liquid line; and the fifth opening and the sixth openingare larger than the second opening or the fourth opening.
 13. The loopheat pipe according to claim 10, wherein the pair of first wallsincludes a first side wall, and a second side wall that is positionedopposite the first side wall; the pair of second walls includes a thirdside wall, and a fourth side wall that is positioned opposite the thirdside wall; the first metallic layer further includes a seventh openingpenetrating the first metallic layer; the second metallic layer furtherincludes an eighth opening penetrating the second metallic layer; theseventh opening and the eighth opening are disposed to coincide witheach other in a plan view; the first opening and the third opening aredisposed adjacent to the first side wall and the third side wall,respectively; and the seventh opening and the eighth opening aredisposed adjacent to the second side wall and the fourth side wall,respectively.
 14. The loop heat pipe according to claim 13, wherein, inthe vapor line, a pillar that supports the uppermost metallic layer isformed by a portion of the first metallic layer disposed between thefirst opening and the seventh opening, and a portion of the secondmetallic layer disposed between the second opening and the eighthopening.